Add missing dividers between examples

Rx.playground/Pages/Combining_Operators.xcplaygroundpage/Contents.swift

@@ -27,6 +27,7 @@ example("startWith") {
 }
 /*:
  > As this example demonstrates, `startWith` can be chained on a last-in-first-out basis, i.e., each successive `startWith`'s elements will be prepended before the prior `startWith`'s elements.
+ ----
  ## `merge`
  Combines elements from source `Observable` sequences into a single new `Observable` sequence, and will emit each element as it is emitted by each source `Observable` sequence. [More info](http://reactivex.io/documentation/operators/merge.html)
  ![](https://raw.githubusercontent.com/kzaher/rxswiftcontent/master/MarbleDiagrams/png/merge.png)
@@ -55,6 +56,7 @@ example("merge") {
     subject2.onNext("③")
 }
 /*:
+ ----
  ## `zip`
  Combines up to 8 source `Observable` sequences into a single new `Observable` sequence, and will emit from the combined `Observable` sequence the elements from each of the source `Observable` sequences at the corresponding index. [More info](http://reactivex.io/documentation/operators/zip.html)
  ![](https://raw.githubusercontent.com/kzaher/rxswiftcontent/master/MarbleDiagrams/png/zip.png)

Rx.playground/Pages/Working_with_Subjects.xcplaygroundpage/Contents.swift

@@ -89,6 +89,7 @@ example("BehaviorSubject") {
 }
 /*:
  > Notice what's missing in these previous examples? A Completed event. `PublishSubject`, `ReplaySubject`, and `BehaviorSubject` do not automatically emit Completed events when they are about to be disposed of.
+ ----
  ## Variable
  Wraps a `BehaviorSubject`, so it will emit the most recent (or initial) value to new subscribers. And `Variable` also maintains current value state. `Variable` will never emit an Error event. However, it will automatically emit a Completed event and terminate on `deinit`.
 */